[meteorite-list] Tektite debate

From: Matson, Robert <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Thu Apr 22 09:43:32 2004
Message-ID: <AF564D2B9D91D411B9FE00508BF1C8698E5556_at_US-Torrance.mail.saic.com>

Hi Steve and Kelly (and whoever else is following this debate),

Steve -- a couple of remarks about some of your arguments. My
apologies for quoting you out of full context, but I believe
the sections I've "snipped" stand on their own, and requoting
two and three times tends to get tedious for readers:

You wrote, "I do not confess to be a physicist or a mathematician such as
yourself-- However-- I do see that your assumptions lack certain elements
that are required to explain obvious facts. That is Tunguska events, and
also
the possibility based on the physical evidence that mega Tunguska events
can,
and do occur-- as the presence of the tektites attest."

You're applying a bit of circular reasoning here. You've been
saying that tektites are one of the resultant effects of a "mega
Tunguska" event, but here you argue that tektites are the proof
that mega Tunguska events do occur.

While the Tunguska event was clearly impressive in terms of the
scale of devastation it caused, physics does not allow you to
linearly "scale-up" this event by much. Beyond a relatively
small scaling factor (I doubt even as much as a factor of 10),
some mass must reach the ground retaining the majority of its
cosmic velocity. I don't see how you can avoid creating both
an atmospheric AND a land crater.

Unless, perhaps, the impact occurred over the ocean. I realize
that tektites are bone dry, but water could not exist in liquid
form at the impact point (and perhaps not even in gaseous
form). Could the ocean floor provide the source of the tektite
silica? If so, could large impact craters remain hidden at
the ocean bottom, or have global surveys today been sufficiently
thorough that none of this size could avoid discovery? (I'm
asking -- I don't know the answer.) I'm just trying to approach
this problem from a logic standpoint:

1. Assume tektites are produced from earth material by impacts.
2. The large extent of tektite distribution fields requires
    the mass of the impacting body to be quite large.
3. Medium-sized comets and meteoroids can "explode" in the
    atmosphere (e.g. Tunguska) without producing a crater.
4. Beyond a certain mass, earth's atmosphere cannot prevent
    some material from reaching the "ground" retaining much of
    its cosmic velocity, regardless of the density or structural
    integrity of the impactor. [While Steve might initially
    disagree with this point #4, I'm hoping he'll key-in on the
    initial phrase "Beyond a certain mass".]
5. The mass required by point #2 exceeds the limiting mass in
    point #4. [ Okay, now Steve can object. ;-) ]
6. A "ground" crater must be produced at the time of impact.
7. The craters associated with the 4 (or is it 5?) major tektite
    strewnfields either have not all been found, or there is
    disagreement over whether the candidate craters that have
    been found are of sufficient size or of the proper age to
    match their associated tektites.
8. There has been insufficient time for geologic processes to
    hide evidence of land craters of the size required by point #2.
9. Large craters in ocean basins remain to be discovered.
    [Conjecture on my part, but considering the recentness of
    discoveries like the Yucatan and Chesapeake Bay impacts, I
    think it's a safe bet.]
10. One or more tektite strewnfields may be associated with
    craters in #9 that remain to be discovered. Indeed, statistics
    suggests that 3 out of 4 strewnfields should be associated
    with ocean impacts.

One geographical corollary that comes from #10 is that if the
tektites of the rarest and most unusual shapes are found closest
to the point of impact, then most of these special forms will
have been lost to the ocean.

I guess I need to hurry and post this before it becomes obsolete!
So many long messages are being exchanged back and forth between
Kelly and Steve that no one else has a chance to chime in before
the emphasis of the discussion has shifted gears... --Rob
Received on Wed 18 Jul 2001 03:25:08 PM PDT